Can't join friends server but others can

I can’t connect to my friends server but other people can. I keep getting Server Not Responding. I have correct version of Garry’s Mod. What to do?
Tried to connect through console too.

Any ideas?

Can you join other servers?

yes

You may need to unblock the port in the router, although technically it should be the same for all servers

Unlock? You mean open? I’ve opened the port 27015 or whatever it was

Still having the problem. I tried disabling all anti-virus stuff and firewall, also opened tons of different ports. No luck.